JavaScript Promises

// A Promise has 3 states: pending → fulfilled OR rejected
const promise = new Promise((resolve, reject) => {
  // Do async work...
  const success = true;

  if (success) {
    resolve("Data loaded!");   // fulfilled
  } else {
    reject(new Error("Load failed")); // rejected
  }
});

promise
  .then(data => {
    console.log(data);  // "Data loaded!"
  })
  .catch(error => {
    console.log(error.message);  // if rejected
  })
  .finally(() => {
    // Runs ALWAYS — success or failure
    // Good for hiding loading spinners
  });

fetch("/api/user/1")
  .then(response => response.json())   // parse JSON
  .then(user => fetch(`/api/posts?userId=${user.id}`))
  .then(response => response.json())
  .then(posts => console.log(posts))
  .catch(err => console.log("Error:", err));

// Each .then receives the return value of the previous

// All requests start at the same time — faster than sequential
const [user, posts, comments] = await Promise.all([
  fetch("/api/user")    .then(r => r.json()),
  fetch("/api/posts")   .then(r => r.json()),
  fetch("/api/comments").then(r => r.json())
]);
// If ANY promise rejects, the whole thing rejects

// Promise.allSettled — get results even if some fail
const results = await Promise.allSettled([p1, p2, p3]);
results.forEach(r => {
  if (r.status === "fulfilled") console.log(r.value);
  if (r.status === "rejected")  console.log(r.reason);
});

// Instantly resolved
const p1 = Promise.resolve("immediate value");

// Instantly rejected
const p2 = Promise.reject(new Error("something went wrong"));

// Useful in tests or conditional logic
function getUser(id) {
  if (!id) return Promise.reject(new Error("ID required"));
  return fetch(`/api/users/${id}`).then(r => r.json());
}